1. Field of the Invention
In the special case where continuous coherent-noise events stand out clearly prior to the arrival of the desired seismic-energy, an adaptive noise cancellation filter (ANC) can be empirically created to provide a reference model of the noise. The undesired continuous coherent noise that contaminates the seismic data is removed by ANC filtering.
2. Discussion of Related Art
In the art of marine seismic geophysical exploration, a plurality of seismic-signal sensors are secured at regular intervals to a long streamer cable of any well-known type which may be 10,000 feet long or more. The streamer cable includes a plurality of electrical-signal communication channels and means for rendering the streamer cable neutrally buoyant. The streamer cable and its attached sensors are towed along a line of survey, just beneath the water surface, typically 30 feet, at a desired constant velocity such as six knots. At timed intervals, such as every six seconds, an acoustic source emits a wavefield that propagates downwardly to insonify the sea floor and sub-bottom earth formations. The wavefield is reflected from the respective formation-boundary interfaces, whence the reflected wavefields return to the near-surface sensors. The variations in mechanical motion or water pressure due to the reflected acoustic waves are converted to electrical analog signals by the sensors. The analog signals are digitized and transmitted through the streamer cable communications links to well-known recording equipment aboard the towing ship. There, the digital data are recorded for archival study as time scale recordings on magnetic tape, diskettes, CD ROM devices or any other state-of-the-art recording/reproducing equipment.
As is well known, when an acoustic source is activated, the first-arriving impulse at the sensors closest to the source travels by a direct path through the water from the source to the sensor following a time delay proportional to the source-sensor distance divided by the water velocity which is about 5000 feet per second. In most cases, the time delay to the nearest sensor may be a few hundredths to a tenth of a second. At increasing sensor distances from the source, the first-arriving seismic signals are increasingly delayed. Sensors from the mid- to the far end-points of a two-mile-long cable may remain quiet for two or more seconds before the desired seismic signals arrive.
All seismic signals contain information of some sort. Sought-after seismic signals are those that can be correlated continuously over extended regions and that provide a quantitative measure of the attributes of sub-ocean earth strata.
Incoherent seismic signals that offer only random, non-continuous bits of information are considered to be noise signals that must be removed from the sought-after signals. Incoherent noise signals originate from sources such as, but not limited to vehicular, pedestrian or bestial traffic, surface water waves, surging of the streamer cable and sparkling noise emanating from virtual point sources located on a rugose sea floor. That sort of noise can usually be removed during routine data processing using well-known destructive random-waveform cancellation techniques.
Noise may also be coherent and correlatable over a limited region but nevertheless undesirable. Such noises include water column reverberation, the regular pulsations of own or other ships' screws, acoustic wavefields generated by a competing geophysical exploration crew, voices of marine life such as whales, noise due to near-surface scatterers or machine noise from offshore oil rigs. Various methods are known for removing such noise including stacking, f-k filtering, array-forming, beam steering and vertical or lateral averaging or a combination of both.
Coherent noise due to competing geophysical exploration ships may be suppressed by reformatting common shot gathers into common receiver gathers as taught by U.S. Pat. No. 4,937,794, issued Jun. 26, 1990 to R. A. Marschall et al. and assigned to the assignee of this invention. Pairs of seismic traces from a common receiver gather are corrected for differential moveout, weighted in inverse proportion to the RMS signal power and combined to generate compressed common receiver gathers. The compressed common receiver gathers may be reformatted as common midpoint gathers for further processing.
U.S. Pat. No. 5,293,352, issued Mar. 8, 1994 to Robert E. Chambers and assigned to the assignee of this invention, teaches a method for removing noise due to near surface scatterers by resorting a set of raw common shot gathers into common receiver gathers. The wavefield envelopes from the common receiver gathers are migrated using one half the near surface velocity to provide migrated data sets. The migrated data sets are resorted back into common shot gathers and are subtracted from the original raw common shot gathers to provide noise-free data sets.
U.S. patent application Ser. No. 08/101,949, filed Aug. 04, 1993 in the name of William H. Dragoset and now U.S. Pat. No. 5,365,492, issued Nov. 15, 1994, assigned to the assignee of this invention discloses an adaptive noise cancellation method for ridding seismic recordings of reverberation noise. The signals detected by a pressure sensor are subtracted from the signals detected by a motion sensor to derive a difference signal. The normalized amplitude of the difference signal is the level of the noise contribution to the total signal. The difference signal is then subtracted from the total signal to reveal the desired noise-free signal.
Other filtering methods that are known include certain image enhancement techniques such as described by J. Besag in On the Statistical Analysis of Dirty Pictures, published in the Journal of the Royal Statistical Society, v. 48, n, 3, 1986.
The first two patents require a considerable quantity of computer resources for implementation. The Dragoset Application not only requires a large amount of computer-aided calculation, but it also requires the use of double the number of sensors normally needed for exploration in order to get an independent reference-noise measurement.
In certain special cases involving continuous, coherent noise such as that due to a passing ship or due to a drilling rig, it is possible to employ a somewhat simpler adaptive noise cancellation process than the ones cited above. There is a need for an adaptive noise cancellation process having the capability to derive a reference model of the noise from data extracted from the seismic shot record itself.